The present invention concerns plasma immersion ion implantation reactors of the type disclosed in the above-referenced parent applications in which a pair of external reentrant conduits define a pair of transverse toroidal paths in which RF oscillating plasma currents are maintained by RF source power applied to the interiors of the conduits. A plasma immersion ion implantation process carried out in such a reactor typically requires that the semiconductor wafer be cleaned or otherwise prepared beforehand. Such a cleaning process can be carried out very rapidly using a plasma process, but this can leave the wafer surface very rough, leading to inferior results. For example, a rough surface that is ion implanted with a dopant material can have an excessive sheet resistance. Deposition of an epitaxial layer (by a plasma enhanced chemical vapor deposition process, for example) on a rough surface can result in a low quality deposited layer that is less crystalline and more amorphous. Such problems can be avoided by carrying out the wafer cleaning process using a cleaning gas without employing any plasma. This approach leaves the smoothest wafer surface but can be unacceptably slow, and in many cases should be carried out at a sufficiently high temperature to activate the gas species. Such a high temperature can exceed the wafer process thermal budget. In order to clean the wafer faster but avoid surface damage of the type encountered with plasma cleaning processes and high temperatures of the non-plasma cleaning process, the cleaning gas can be dissociated into reactive neutrals. This latter approach would be ideal, except that it requires a remote plasma source (RPS) reactor that furnishes the reactive neutrals or radicals to the main chamber. The problem is that the external feed from the RPS chamber to the main chamber holding the wafer does not provide a uniform distribution of the neutrals or radicals over the wafer surface, so that the wafer cannot be cleaned uniformly. Typically, the radicals or neutrals from the RPS chamber are fed to a side port of the main chamber, leading to the non-uniformity. What is needed is a way of cleaning a wafer rapidly and uniformly without impairing wafer surface quality and without having to employ very high temperatures.
A plasma immersion ion implantation reactor must be cleaned periodically. In some cases, such periodic cleaning is required to avoid excessive metal contamination of the wafer process. The best results (lowest contamination) are obtained when the chamber is cleaned every time a wafer is processed. This is only practical if the cleaning process is less than the time required to perform the wafer process, in order to avoid an excess loss of productivity. The fastest processes for cleaning the chamber interior are plasma cleaning processes, and these can meet the productivity goals. Unfortunately, plasma cleaning process are so fast that they tend to consume a relatively large fraction of the chamber surfaces and elements (such as process kits), and therefore are extremely costly insofar as they require frequent replacement of chamber interior parts and materials. The minimum consumption of chamber interior elements for a thorough cleaning is obtained using cleaning gases without a plasma, but this approach is too time-consuming. The best compromise is obtained by employing dissociated cleaning gases. The problem with this approach is that distribution within the main chamber of the dissociated cleaning gases from an external remote plasma source (RPS) chamber is non-uniform, so that the main chamber cannot be cleaned uniformly. This is because the external feed from the RPS chamber to the main chamber does not provide a uniform distribution of the neutrals or radicals within the main chamber. Typically, the dissociated gases from the RPS chamber are fed to a side port of the main chamber.
One problem encountered in plasma immersion ion implantation is that the dopant-containing process gas can sometimes form a film on the surface being implanted that can block the implantation or distort the implant depth profile from the desired one. Such an unwanted film can, in some cases, distort the ion implantation depth profile (or render it difficult to control during implantation), so that the resulting depth profile may not be ideal. Another problem is that the ion bombardment can etch away the surface being implanted, removing much of the implanted ions and thereby attenuating the desired effects of the implantation process. In a dopant implantation process in a semiconductor layer, this problem manifests itself as a high sheet resistance.
Another problem that can arise in any plasma process is contamination on the wafer backside that degrades subsequent wafer processing steps. Our experience has led us to believe that such backside contamination arises from contact between the wafer backside and the electrostatic chuck (ESC) top surface and the flexing of the wafer during wafer chucking and dechucking. Metallic contamination occurs because in many cases the insulating layer on the ESC surface is a metal containing compound such as AlN. AlN particles scraped onto the wafer backside from the ESC can be dissociated in later plasma process steps to free the Al species and form metallic contamination, which can degrade process performance. There is a need to prevent such contamination without creating other burdens on the process.